1. Field of the Invention
The present invention relates to a color toner for developing electrostatic images used in forming an image by electrophotography, an electrostatic latent image developer, and an image forming method.
2. Description of the Related Art
At present, the method of visualizing image information via an electrostatic image by an electrophotographic method etc. is used in various fields. In the electrophotographic method, an electrostatic image is formed on a photoreceptor in a light exposure step, and the electrostatic latent image is then developed with a toner-containing developer and visualized through transferring and fixing step. The developer used herein includes a two-components developer comprising a toner and a carrier and a one-component developer using a magnetic or non-magnetic toner alone, and the toner is produced usually by a kneading and milling process wherein a thermoplastic resin is melt-kneaded with a pigment, a charge controlling agent, and a releasing agent such as wax, then cooled, pulverized, and classified. Fine inorganic and organic particles for improving fluidity and cleaning properties may be added if necessary to the surfaces of such toner particles.
Copiers by a color electrophotographic method, printers, or combined machines thereof such as facsimiles have been widely distributed in recent years, but gloss suitable for reproduction of color images and transparency for achieving excellent OHP images are hardly realized by a toner containing a releasing agent such as wax, prepared by melt kneading. This is because wax such as polyethylene, polypropylene and paraffin used generally in usual black and white copies is melt-kneaded to permit the domain diameter of the releasing agent to be varied, and when such toner is used, the transparency of OHP images is deteriorated. Accordingly, a large amount of oil is applied onto fixing rolls to facilitate release, but causes stickiness of reproduced images including OHP images and makes writing on the images by a pen difficult, and uneven gloss may often occur.
Further, the method of producing a toner by the conventional kneading and milling process hardly prevents the releasing agent from being exposed to the surface of the toner, so that when the toner is used as a developer, there arise additional problems such as a significant deterioration in fluidity, filming on a developing unit and a photoreceptor, etc.
As a method of essentially solving these problems, there is a proposal on a polymerization process wherein a toner is produced by dispersing an oil phase composed of a monomer as a starting material of resin and a colorant in an aqueous phase and then polymerizing the monomer directly thereby allowing the wax to be included in the toner to control exposure of the wax to the surface.
As another means of enabling intentional regulation of the shape and surface structure of a toner, a method of producing a toner by an emulsion polymerization aggregation method is proposed. This is a production process which comprises preparing a dispersion of fine resin particles generally by emulsion polymerization while separately preparing a dispersion of colorant particles having colorant particles dispersed in a solvent, mixing the dispersions to form aggregated particles having a size corresponding to the particle size of an intended toner, and coalesced the particles by heating to form the toner.
The above-described process for producing a toner not only realizes inclusion of wax but also facilitates formation of a toner of smaller diameter to achieve reproduction of vivid images of higher resolution, and there is demand for further improvements (see Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439).